Supermagnetostrictive alloy and method of preparation thereof

ABSTRACT

The present invention provides a supermagnetostrictive alloy capable of providing a larger shift (lager magnetostriction) with excellent workability, which is applicable to an actuator in response to advances in downsizing of electronic devices and upgrading of medical instruments and production apparatuses. The supermagnetostrictive alloy has a degree of order of 0.6 to 0.95 achieved by subjecting Fe 3−x Pt 1+x (−0.02≦×≦0.2) to a heat treatment. The present invention also provides a method for the preparation of a supermagnetostrictive alloy having a magnetostriction of 0.3% or more, particularly 0.5% or more, comprising the steps of subjecting the Fe 3−x Pt 1+x  alloy of a raw material to a homogenization annealing, and then subjecting the resulting product to a heat treatment at 700 to 1000 K for 0.5 to 600 hours.

TECHNICAL FIELD

The present invention relates to a supermagnetostrictive alloy or amagnetic alloy capable of exhibiting a giant magnetostriction through aphase transformation, and a method for preparation thereof.

BACKGROUND ART

A functional material usable as a member for generating a shift(magnetostriction) and a force (stress) is referred to as actuator,which is used in a wide range of fields such as electronic devices,medical instruments and production apparatuses. Such a functionalmaterial includes a piezoelectric material, a magnetostrictive material,a shape-memory material and others. One of notable magnetostrictivematerials is a rare-earth alloy Tb_(0.3)Dy_(0.7)Fe (Journal of Alloysand Compounds, vol. 258, 1997), which has been commercialized (TradeName: Terfenol-D). Terfenol-D has a maximum magnetostriction of 0.17%.

Japanese Patent Laid-Open Publication No. 11-269611 discloses aFe/Pt-based or Fe/Pd-based rapidly solidified alloy having amagnetostriction of 0.15 to 0.2%. Further, Ni₂MnGa is known as ashape-memory alloy whose crystal state is changed by magnetic field orelectric field to provide a high-speed activation (Industrial MaterialsVol. 45, No. 12, Nov. 1997, pp 108-111, Japanese Patent Laid-OpenPublication No. 10-259438). However, this shape-memory alloy hasmechanical brittleness and poor workability. Japanese Patent Laid-OpenPublication No. 62-170453 discloses another shape-memory alloy whichcontains 25-30 at % of Pt added into Fe and has an irregular atomicarrangement to provide enhanced workability. Japanese domesticpublication of PCT application in Japanese language No. 11-509368discloses a method for controlling a material having a twin structure tocause change in shape and generate movement and/or force in the materialby applying to the material a magnetic field having a directionality andmagnitude suitable for achieving a desired reorientation of the twinstructure.

Problem to be Solved by the Invention

In line with advances in downsizing of electronic devices and upgradingof medical instruments and production apparatuses, it is desired toachieve an actuator made of a material capable of providing a largershift (lager magnetostriction) with excellent workability.

DISCLOSURE OF THE INVENTION Means for Solving the Problem

With focusing on a phase transformation in an alloy structure, theinventors has successively achieved a magnetostriction of 0.3% or more,particularly 0.5% or more by subjecting Fe_(3−x)Pt_(1+x)(−0.02≦×≦0.2) toa heat treatment under inventive conditions for ordering.

Specifically, the present invention is directed to asupermagnetostrictive alloy having a degree of order of 0.6 to 0.95achieved by subjecting Fe_(3−x)Pt_(1+x)(−0.02≦×≦0.2) to a heattreatment. Even if the ordering is performed to a Fe_(3−x)Pt_(1+x) alloyhaving less than −0.02 or greater than 0.2 of x, no FCC-FCT martensitictransformation is caused. A preferable range of x is 0.0≦×≦0.1.

Further, the present invention is directed to a method for thepreparation of the above supermagnetostrictive alloy comprising thesteps of subjecting a Fe_(3−x)Pt_(1+x)(−0.02≦×≦0.2) alloy of a rawmaterial to a homogenization annealing, and then subjecting theresulting product to a heat treatment at 700 to 1000 K for 0.5 to 600hours. Given that when all of Pt and Fe in Fe₃Pt having a face-centeredstructure are accurately arranged at corners and face-centers of thecrystal structure, respectively, such a state is defined as a fullyordered state or the degree of order S=1, a maximum magnetostrictioncould be achieved by arranging the degree of order in the range of 0.6to 0.95. When the degree of order is less than 0.6 or greater than 0.95,no FCC-FCT martensitic transformation is caused.

The alloy of the present invention exhibits an extremely largemagnetostriction of 0.5% even under a weak magnetic field of about 4 T.As for magnetostrictive alloys, by extension of a conventionalconception in which spins are coordinated with each other in a magneticdomain, an obtainable magnetostriction Δ I/I is the order of 10⁻⁶ atutmost. One of the main reasons for the extremely large magnetostrictionof 0.5% (5×10⁻³) in the present invention is that a crystallographicdomain (variant) is conformed to a magnetic domain to form a singledomain and thereby spin axes are crystallographically coordinated witheach other as well.

The present invention provides an alloy having excellent workability andthereby the alloy can be formed in a single crystal bulk, polycrystalbulk, thin sheet shape (including roll shape), linear shape, thin filmshape or the like. The present invention can also provide a shape-memoryalloy in which the crystallographic domain (variant) and magnetic domainare approximately equalized in size and aligned in the direction of anapplied magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a phase transformation based on an orderingtime as a function.

FIG. 2 is a graph showing the relationship between an applied magneticfield and a magnetostriction of a Fe₃Pt single crystal in a firstexample.

FIG. 3 is a graph showing a temperature dependence of the magneticsusceptibility of the Fe₃Pt single crystal in the first example.

FIG. 4 is a diagram showing X-ray profiles of the Fe₃Pt single crystalin the first example at 77 K and 100 K.

FIG. 5 is a graph showing a temperature dependence of the thermalexpansion coefficient of the Fe₃Pt single crystal in the first example.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, a Fe_(3−x)Pt_(1+x)(−0.02≦×≦0.2) alloy as a rawmaterial or un-heat-treated material can be prepared through a singlecrystal production method, a casting method, a sputtering method or thelike. The raw material is subjected to a homogenization annealing atabout 1200 to 1700 K, and the homogenization-annealed raw material iscut into a given shape. Then, the raw material is subjected to asolution treatment at about 1200 to 1700 K to remove the distortion dueto the cutting. For an ordering, the solution-treated raw material isthen enclosed in a silica tube which is held with vacuum or filled withAr gas to prevent oxidation of the raw material, and is heated at 700 Kto 1000 K, preferably 800 K to 900 K for 0.5 to 600 hours., preferably 1to 96 hours.

Any suitable speed may be selected to heat the raw material up to theordering temperature and cool the heated material down from the orderingtemperature. The heated material may be cooled down by either one ofair-cooling and water-cooling (for example, a process of immersing intowater having a temperature of 20° C.). The product of the orderingtemperature x the ordering time is adjusted to obtain a desired degreeof order S ranging from 0.6 to 0.95. When the raw material is preparedthrough the casting method, it is desired to forge or roll the rawmaterial so as to coordinate the crystal orientation. In an alternativeembodiment, an iron-platinum alloy may be deposited on a substrateheated up to 770 to 1000 K.

Through the ordering, the crystal structure of the raw material ischanged from a face-centered cubic (FCC) structure to a face-centeredtetragonal (FCT) structure. Thus, the degree of order can be detected byan X-ray analysis. FIG. 1 is a graph showing a phase transformationbased on an ordering time in the ordering at 923 K. FIG. 1 shows that amartensitic transformation (Ms) temperature of BCT martensite (solidline) and FCT martensite (broken line) and an initiation temperature ofthe formation of a tweed pattern in the crystal structure depend on theordering time. The FCT region shown on the lower right side of FIG. 1corresponds to the alloy of the present invention having a degree oforder of 0.6 to 0.95.

FIRST EXAMPLE

A Fe₃Pt single crystal bulk alloy was prepared through Floating Zonemethod and processed as follows.

(1) Melting: 26 g of iron and 30 g of platinum were molten in anarc-melting furnace to provide an iron/platinum atomic ratio of 3:1.

(2) Single-Crystallization: Using a four-ellipsoidal-mirror typeFloating Zone melting apparatus, the molten iron/platinum wassingle-crystallized through Floating Zone method to obtain a Fe₃Ptsingle crystal bulk of 56 g.

(3) Homogenization Annealing: Using an electric furnace, the Fe₃Ptsingle crystal bulk was heated at a constant temperature of 1373 K for24 hours, and then slowly cooled.

(4) Cutting: The heat-treated single crystal bulk was cut into a cubehaving a side of 2 mm. The crystal orientation of the cut single crystalbulk was checked through a Laue method and adjusted to match thedirection of [001] with the direction of a magnetic field.

(5) Enclosing in Silica Tube; Then, the cut single crystal bulk wasenclosed in a silica tube having a diameter of 10 mm and a length of 50mm.

(6) Solution Treatment: In order to remove the distortion due to thecutting, the single crystal bulk was heated at a constant temperature of1373 K for 1 hour in the silica tube held with vacuum or filled with Argas, and then slowly cooled.

(6) Ordering: Using an electric furnace, the single crystal bulk washeated at a constant temperature of 923 K for 12 hours.

(7) Cooling: After heating, the single crystal bulk was air-cooled.

Measurement of Magnetostriction: The thermal expansion andmagnetostriction were measured by detecting change of the distancebetween two planes through a capacity measurement. A three-terminalcapacitance method was used for the capacity measurement. For the singlecrystal of two cubic millimeters, the thermal expansion andmagnetostriction were measured while matching the direction of [001]with the direction of a magnetic field.

FIG. 2 shows the measurement result. The magnetic field intensity (T)was changed from 0 T to 4 T as shown by the code {circle around (1)}(solid line), from 4 T to 0 T as shown by the code {circle around (2)}(one-dot chain line), and from 0 T to −4 T and further to 0 T as shownby the code {circle around (3)} (two-dot chain line). As can be seenfrom FIG. 2, the Fe₃Pt single crystal of the present invention exhibitsa magnetostriction of 5×10⁻³. This value is equivalent to three-times ofthat of conventional examples. FIG. 3 shows a temperature dependence ofthe magnetic susceptibility, and FIG. 4 shows X-ray profiles at 77 K and100 K. Based on FIGS. 3 and 4, the number of domains and theirmagnetostrictions can be calculated, and these calculated values areconsistent with the measurement result. FIG. 5 is a graph showing atemperature dependence of the thermal expansion coefficient. As can beseen from FIG. 5, a shape change of 5×10⁻⁵ is exhibited, and threevariants are introduces.

SECOND EXAMPLE

A Fe₃Pt polycrystal bulk sheet-shaped alloy was prepared through acasting method and processed as follows.

(1) Melting: The same molten metal as that in the first example wassolidified in a water-cooled copper crucible to obtain a Fe₃Ptpolycrystal bulk having a size of 30×20×10 millimeter.

(2) Rolling: The polycrystal bulk was rolled through a single-roll driverolling method to form a sheet-shaped sample having a thickness of 1 mm.

(3) Homogenization Annealing: This step and subsequent steps are thesame as those in the first example.

The magnetostriction of the obtained alloy was 3×10⁻³.

THIRD EXAMPLE

A Fe₃Pt polycrystal alloy was prepared through a spattering method andprocessed as follows.

(1) Spattering: Using a magnetron-spattering apparatus, an alloy havinga composition of Fe₃Pt as a target was spattered on a substrate toprovide an iron/platinum atomic ratio of 3:1. Then, a Fe₃Pt alloy filmhaving a thickness of 0.001 mm was obtained from the deposited filmpeeled from the substrate.

(2) Homogenization Annealing: This step and subsequent steps are thesame as those in the first example.

The magnetostriction of the obtained alloy film was 3×10⁻³.

INDUSTRIAL APPLICABILITY

The magnetostrictive alloy of the present invention exhibits anextremely large magnetostriction of 0.3% or more, particularly 0.5% ormore, and has excellent workability. Thus, the alloy can be formed asvarious forms such as sheet members, foils, or wires. The alloy can alsobe formed as a thin film through a spattering method.

What is claimed is:
 1. A method for the preparation of asupermagnetostrictive alloy having a degree of order of 0.6 to 0.95,said method comprising the steps of: subjecting a Fe_(3−x) Pt_(1+x)alloy of a raw material to a homogenization annealing, where the rangeof x is −0.02≦×≦0.2, and subjecting the Fe_(3−x) Pt_(1+x) alloy to aheat treatment at 700 to 1000 K for 0.5 to 600 hours.
 2. The method forthe preparation of a supermagnetostrictive alloy according to claim 1,wherein the heat treatment is at 800 to 900 K.
 3. The method for thepreparation of a supermagnetostrictive alloy according to claim 1,wherein the heat treatment is for 1 to 96 hours.
 4. The method for thepreparation of a supermagnetostrictive alloy according to claim 1,wherein the heat treatment is at 800 to 900 K for 1 to 96 hours.
 5. Themethod for the preparation of a supermagnetostrictive alloy according toclaim 1, wherein the homogenization annealing is at 1200 to 1700 K.